I am beginning to attract some religious conspiracy theorists…I think I’d better change the subject!
So…deep breath. I’m going to attempt to explain this whole Higgs boson thing which the news keeps going on about, and which, seeing as it is supposedly one of the most important things ever, I’ve been meaning for a while to actually try to properly understand. Usual disclaimers: I am not a physicist (in fact whether or not I’m even a proper mathematician is arguable) and I am writing this mainly as a motivation to increase my own understanding. However, my theory is that, unless an expert is a supremely good communicator, it is often easier to gain a basic understanding of a complex subject from another interested layperson (as they know exactly how you feel). Certainly I would have liked someone else to have written something like this to save me the effort!
I think we have all heard about the search for the Higgs boson by the people at CERN. Probably, if you’re still reading this, you have also, like me, wondered exactly what this boson is, what it does, and why it matters so much. And probably you have some vague notion that it is a particle which “gives other particles mass”. That is the point I shall start from.
But first, a question – why are things the size they are? Sounds a bit vague and philosophical, I know. But the size of an object is determined by the size of the molecules which make it up, which are in turn determined by the size of their constituent atoms. Atoms consist of a nucleus made up of protons and neutrons, surrounded by orbiting electrons. And the size of an atom is determined by the sizes of the orbits of its electrons. But the size of electrons’ orbits depends on the mass of the electron! So in order to find an answer to why things are the size they are, we need to address the question of why an electron has the mass it does. And while we’re at it, we may as well ask why other elementary particles have the mass they do…for example, why do photons have no mass at all?
So there’s a bit of motivation. Of course, the question of why mass exists (which is really what we’re asking when we ask why fundamental particles – which are the building blocks of the universe – have the mass they do) belongs in the realm of philosophy. However, we can at least use scientific methods to approach the question of how there comes to be mass. The problem is that the simplest mathematical theory (and it is usually the simplest ones that are right) says that all fundamental particles should be entirely massless, which is clearly wrong.
Now, physics proceeds as follows: theoretical physicists use experimental evidence to formulate mathematical theories, and experimental physicists attempt to either verify or disprove these theories, in a constant back and forth. It is not necessarily for the best when the evidence is in favour of a theory though; all this does it to make that theory a bit more likely. On the other hand, just as in mathematics, it only takes one counterexample to completely rule something out. It is often when a particularly well-established theory is shown by an experiment to be “false” that new discoveries are made: for example, Wolfgang Pauli invented the neutrino in a “desperate” (his own word) attempt to explain away the apparent violation of conservation of energy in radioactive beta decay; he was later shown to be correct.
In the case of the mass of particles, we have something of a reverse situation: a well-established empirical observation (things have mass!) which is contradicted by what seems to be the obvious theory. And so, in time-honoured tradition, Peter Higgs postulated away this discrepancy with his proposal of a new particle, which has come to be known as the Higgs boson (a boson, by the way, is just a type of subatomic particle characterised by the fact that it has integer spin…”spin” is just some kind of odd quantum-mechanical version of exactly what it sounds like, which I won’t go into).
Now, it is by no means certain that the Higgs boson exists; as you may have gathered, the point of the Large Hadron Collider is largely to clear this issue up. By accelerating particles to obscene speeds and crashing them into each other we can produce a shower of smaller particles, and it is hoped that eventually the Higgs boson will be spotted in one of these showers. However, although the Standard Model (currently the most widely accepted “theory of just about everything”) does predict the existence of the Higgs boson, it unfortunately does not predict the mass of the Higgs boson, hence the protracted search. Of course, the Standard Model is just a model, and the Higgs boson might not exist at all. This wouldn’t be such a bad thing (unless perhaps you are Peter Higgs), as there are lots of alternative theories as to how particles gain mass (known as Higgsless models, which sound delightfully like something out of Hundred-Acre Wood).
But it is generally accepted that the most likely scenario is that it does exist. In which case, how does it work? Well, the reason quantum theory has its name is that it was observed that certain physical properties change only in discrete “quanta”, rather than continuously. One of the most central concepts in quantum theory is that of “wave-particle duality”, which says that objects at the quantum scale exhibit characteristics of both continuous waves and discrete particles. Similarly, quantum field theories like the Standard Model say that waves in the supposedly continuous fields of classical mechanics, such as the electromagnetic and gravitational fields, are in fact quantised, and it is the quantum “excitations” (energy levels) of these waves which are the elementary particles.
That is, every field has a particle associated with it, and the force exerted by a given field can be viewed as the action of its “force carrier” particles. For example, the particle associated with the electromagnetic field is the photon; as such all flux in the electromagnetic field has discrete levels, with the basic unit of flux a photon. It has even been postulated that there is an as-yet-undiscovered particle – the graviton – which mediates the gravitational field (the Standard Model does not explain gravity, among some other things, hence the “just about” qualification above). This differs from the classical view of particles with forces acting between them, as it implies that in fact everything is particles…for example, the electromagnetic “force” between two electrons is simply an exchange of photons. Alternatively, everything is really just fields: particles are not little balls at all, but simply discrete fluctuations in a field. It’s all a bit confusing, but however you look at it, fields are particles are fields…
So, the fabled Higgs boson, being an elementary particle, has its very own associated field. The Higgs field exists throughout the universe, in the vacuum between every particle, and it is the interactions of particles with this field which is postulated to give them their mass. That is, we can think of particles moving through the Higgs field: the more they interact, or are slowed down by the field, the greater their mass. The theory goes that, in the Beginning, every particle travelled at the speed of light, and had no mass. And then God said “Let there be mass”, and lo, they started to interact with the Higgs field, like so many balls of various stickiness moving through a pool of molasses. Some, such as photons and neutrinos, simply shrugged off the molasses and zipped through on their merry way, to be forever massless and light-speed. Others became bogged down and sluggish, and it is this very sluggishness that we perceive as mass.
That probably isn’t very satisfactory…I haven’t even addressed how the Higgs field interacts with the particles. But my brain is tired, and it’s about as far as I can go without getting into some pretty heavy duty stuff. So I’ll finish by quoting an analogy of how the Higgs mechanism works, using Margaret Thatcher as the analogue of an elementary particle, and a roomful of lesser politicians as the Higgs field. This was written by one Professor David Miller for the UK Science Minister in 1993, in terms I suppose he thought the minister might understand:
Imagine a cocktail party of political party workers who are uniformly distributed across the floor, all talking to their nearest neighbours. The ex-Prime- Minister enters and crosses the room. All of the workers in her neighbourhood are strongly attracted to her and cluster round her. As she moves she attracts the people she comes close to, while the ones she has left return to their even spacing. Because of the knot of people always clustered around her she acquires a greater mass than normal, that is, she has more momentum for the same speed of movement across the room. Once moving she is harder to stop, and once stopped she is harder to get moving again because the clustering process has to be restarted. In three dimensions, and with the complications of relativity, this is the Higgs mechanism…
(Note that momentum is simply mass times velocity. If a particle gains momentum, but remains at the same velocity, then it gains mass.)
…Now consider a rumour passing through our room full of uniformly spread political workers. Those near the door hear of it first and cluster together to get the details, then they turn and move closer to their next neighbours who want to know about it too. A wave of clustering passes through the room. It may spread out to all the corners, or it may form a compact bunch which carries the news along a line of workers from the door to some dignitary at the other side of the room. Since the information is carried by clusters of people, and since it was clustering which gave extra mass to the ex-Prime Minister, then the rumour-carrying clusters also have mass. The Higgs boson is predicted to be just such a clustering in the Higgs field.